I don't think anyone was tryin to substitute midrange speakers for sub bass...Most people want to replace their scratch paper oem junk...we help with the sizes and hope they audition speakers to pick the right brand.
I've seen 4 15's in a jeep each with their own amp...profile on car domain actually saw the vehicle in Miami. There are a few jks with fiberglass craziness that made the show circuits in 2008ish. It's usually overkill...most listeners would be fine with an 8" near them (look at how many people are happy with their upgraded center console sub. Others have no interest or money for a sub...for them spending money on a quality set of midrange speakers is the best route.
For those who want more, a good quality 10 or 12 in the right enclosure backed with rms at the high end of the subs handling will hit harder than expected for a jeep.
The reason to pair the subs with a decent front stage; 5.25's, amped is better is because if you're investing all that money and running quality threads you might as well have some lyrics with your bass.
I guess it depends on whether you dig SQL or spl.
iPhone app user; excuse typos...thanx.